Air compressor systems typically utilize a valve, commonly referred to as an unloader valve, to temporarily allow air that is compressing under normal start-up conditions to vent directly to the atmosphere instead of into an air storage tank to reduce the load initially applied to the motor and compressor. The spring-biased stem of a typical unloader valve is mechanically activated by a pressure switch which is part of the total compressor system. The pressure switch activates the unloader valve at initial start-up or during steady state off conditions when the pressure in the air storage tank rises above a predetermined level.
Two air compressor systems that utilize a solenoid to actuate an unloader valve are disclosed in the prior art references discussed hereafter. U.S. Pat. No. 2,462,232 to Stein discloses an air compressor system having a control means which utilizes the voltage across the capacitor of a single-phase motor to actuate an unloader valve. Referring to the only Figure disclosed in this reference, a motor 1 directly drives a compressor 2 and a speed responsive switch 8 via motor shaft 10. When the motor runs below a predetermined speed, there is no load on the motor 1 because a switch 8 electrically connects the capacitor 7 to the solenoid 14 thereby opening a valve 13 and venting the compressed air to atmosphere. During this time, the switch 8 also energizes the auxiliary winding 3. When the motor reaches the predetermined speed, switch 8 disconnects the auxiliary winding 3 from an external power source and disconnects solenoid 14 from capacitor 7 thereby closing valve 13 and allowing the tank (not shown) to fill with compressed air.
U.S. Pat. No. 2,629,537 to Graybrook et al. discloses a system for controlling the operation of electric motors which drive fluid compressors. When the air pressure in the main reservoir 11 drops below a predetermined level, the pressure switch 16 closes a battery circuit which energizes the motor 14. Simultaneously, the coil 55 of the magnet valve 17 is energized which actuates the valve member 56 and allows the air in the auxiliary reservoir 24 and the valve lifter 41 to be exhausted through a restricted opening 59 in the magnet valve 17. The motor is allowed to reach full speed before the compressor 10 begins pumping against the head pressure in the main reservoir 11.
Air compressor systems that utilize a solenoid to actuate an unloader valve are expensive to manufacture, maintain and use. For example, not only are the material costs increased due to providing a solenoid activated unloader valve and the electrical connections which provide power to the solenoid, but also labor costs are increased due to the time required for mounting and interconnecting the various parts together. Additionally, for example, maintenance and use costs also are increased due to the provision of the additional moving parts.